OK, I know, I've been here before; some people never learn. Last couple years I was busy remodeling, then two grandsons born 2K mi. & 6K mi. away. So my 40 y.o. A4 reminds me about the true cost of neglect. Picture the corroded valve seats and ex.manifold, and its stud. New valves, keepers, washers and assorted gaskets should arrive from M.M. tomorrow. Question is, should I just 'lap the hell out of it' as my machinist neighbor said last go 'round? Or is it time to face the music, pull the engine, and have the machine shop regrind the seat in 'proper' fashion? And only two studs came free when I pulled the head; the corroded back manifold stud (top) did not. I'm inclined to give the in-the-boat DIY another go, but I'm open to suggestions from wiser folk than I. Thank you.

That water injection point needs to be at least 3~4 inches further downstream.
You could plug it and add the MMI injection Tee below the end of the elbow.
Also check the manifold to see that it has no internal leaks as well.

Dave Neptune

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Sorry, but your neighbor is right. Grind or cut the seats. Sometimes this can be done in-situ. It was called "field service" in the old days. It'd be lovely if you found a machinist to come to tbe dock for a moonlight call.

Last engine I did, the exhaust seats weren't as bad as yours, yet the shop installed carbide inserts (no extra charge). The intake seats cleaned up OK.
That corrosion of the manifold is likely due to internal water, but could also be a leak at one of the manifold studs where it goes into the block. Pulling those studs would give you two things: a) make sure they're sealed well, and b) check the wall thickness of the block in that area.

Thanks to all who have weighed in. My next steps will include reworking the exhaust hot section to reduce the splashback/moisture issues that appear to be the primary cause of internal corrosion of the manifold and #4 valves. I'll replace the third manifold stud and look for internal leaks. And I'll regrind those seats by hand (to avoiding pulling the motor), unless guide wobble indicates a deeper problem. I've also identified and am addressing a leak in the heat exchanger that could be the primary source of external corrosion issues that became apparent earlier. Thanks again.

I'll go on record as saying you should have an experienced hand oversee or do the grinding. It is not an academic exercise—there is an art to it. Best to learn on a disposable engine like a Chevy. You can put too much sideways pressure on the stone and get the seat out concentric alignment with the guide. This is not readily apparent until the pre-assembly or checks with Prussian blue.(https://www.motorcycleproject.com/text/vlvjob04.html)

If you choose grinding over cutting, you will make a lot of grit dust. It will go all over the engine. This stuff is ABRASIVE. Clean, clean, clean.

Yeah, best to be careful. Easy to dive into a project like this and make it worse.
Friend had problems with a knee (ACL?) and was working with a surgeon, who was doing multiple tests. Friend couldn't understand all the testing - just get in there and fit the d... thing! Surgeon explained: Before I go in there, I want to feel confident I can make it better, not worse.
Repairs with the engine in the boat are to get you through the season - maybe. Pulling the engine and doing things right are for the next 20 years.